Fossil fuels currently supply the majority of the world's energy needs and their combustion is the largest source of anthropogenic carbon dioxide emissions. Carbon dioxide is a greenhouse gas and is believed to contribute to global climate change. Concern over global climate warming has led to interest in capturing CO2 emissions from the combustion of fossil fuels. CO2 can be removed from combustion flue gas streams by varying methods.
Combustion gases vary in composition depending on the fuel and the conditions of combustion. The combustion gases can be produced in furnaces and in gas turbines, including the combustion gases produced in the generation of electric power. The fuels used are predominantly coal and natural gas. Coal is burned in furnaces, while natural gas is burned both in furnaces and in gas turbines, but in electric power generation natural gas is mainly burned in gas turbines.
The quantities of combustion gas produced in electric power generation are very large because of the scale of furnaces and turbines used. One measure of the scale of these operations is the amount of CO2 produced in a typical 500 Megawatt power plant. For coal fired power generation, the rate of CO2 production is on the order of 100 kilograms per second; for gas fired power production it is more like 50 kilograms per second.
The challenge for flue gas CO2 capture is to do it efficiently to minimize the cost. All post-combustion CO2 capture technologies suffer from the disadvantage that the CO2 in the flue gas is present at low pressure (just about 1 atm) and in low concentrations (3 to 15%). A large amount of energy is needed to separate the CO2. For 90% recovery of 10% CO2 in a flue gas at 1 atm, the CO2 must be brought from 0.1 atm to 1 atm, and then further compressed to a delivery pressure of 150 atm. Analyses conducted at NETL shows that CO2 capture and compression using a conventional absorption process raises the cost of electricity from a newly built supercritical PC power plant by 86%, from 64 cents/kWh to 118.8 cents/kWh (Julianne M. Klara, DOE/NETL-2007/1281, Revision 1, August 2007, Exhibit 4-48 LCOE for PC Cases). Aqueous amines are considered a state-of-the-art technology for CO2 capture for PC power plants, but have a cost of $68/ton of CO2 avoided)(Klara 2007, DOE/NETL-2007/1282). Developing methods that minimize the amount of energy and other costs will be necessary if CO2 removal from flue gas is to be economical.
Methods for the removal of CO2 from gas streams, include absorption with a solvent, adsorption with a sorbent, membrane separation, and cryogenic fractionation and combinations thereof. In absorption/adsorption processes to capture CO2, the energy needed to regenerate the sorbent or solvent is a large cost element.
The heat of adsorption is generally lower than the heat of absorption. This could make use of physical adsorbent for CO2 capture attractive because it has a lower energy requirement for the desorption reaction to release the CO2. A physical adsorbent can be used for CO2 capture. Using molecular sieves/zeolites and activated carbon, this approach for CO2 capture has been research by Inui 1988 [Inui, T., Okugawa, Y. and Yasuda, M. (1988), Relationship between properties of various zeolites and their CO2 adsorption behaviours in pressure swing adsorption operation, Industrial & Engineering Chemistry Research, 27, 1103], Chue 1995 [Chue, K. T., Kim, J. N., Yoo, Y. J., Cho, S. H. and Yang, R. T. (1995), Comparison of activated carbon and zeolite 13X for CO2 recovery from flue gas by pressure swing adsorption, Industrial & Engineering Chemistry Research, 34 (2), 591-598], Siriwrdane 2001 [Siriwardane, R. V., Shen, M., Fisher, E., and Poston, J. A., (2001) “Adsorption of CO2 on Molecular Sieves and Activated Carbon,” Energy and Fuels, Vol. 15, pp. 279-284, 2001], Siriwrdane 2005a [Siriwardane, R. V., (2005a) “Solid Sorbents for Removal of Carbon Dioxide from Low Temperature Gas Streams”, U.S. Pat. No. 6,908,497 B1, Jun. 21, 2005], Siriwrdane 2005b [Siriwardane, R. V., Shen, M., and Fisher, E., (2005b) “Adsorption of CO2 on Zeolites at Moderate Temperatures,” Energy and Fuels, Vol. 19 No. 3, p. 1153, 2005], Muñoz et al (2006) [Muñoz, Emilio, Eva Díaz, Salvador Ordóñez, and Aurelio Vega “Adsorption of Carbon Dioxide on Alkali Metal Exchanged Zeolites”], Gingichasvili (2008) [Gingichashvili Sarah (May 19, 2008)—http://thefutureofthings.com/news/1183/co2-absorption-made-easier.html], Halmann and Steinbger (1999) [Halmann, M M and M Steinberg, (1999) Greenhouse Gas—Carbon Dioxide Mitigation: Science and Technology. Lewis Publishers, Boca Raton, Fla.] and Lee (2005) [Lee, Sunggyu (2005) Encyclopedia of Chemical Processing Vol 1. CRC Press].
Many physical adsorbent separation systems use pressure swing absorption (PSA) for regeneration. PSA can be used to regenerate CO2 adsorbents. It is used in environmental control applications to maintain CO2 level (Lee 2005) Also, PSA is used for removal of CO2 down to very low levels in gas purification (U.S. Pat. No. 5,656,064). Applying PSA to atmospheric flue gas separation would have high energy consumption requirements (due the required to pull a hard vacuum when removing CO2 from a flue gas) and capital costs because of the large pressure ratios required to enable complete desorption of the CO2.
Temperature swing absorption is another well-established regeneration method. This has been applied to adsorbent regeneration by Lee (2005).
Such separation processes are also commonly applied for CO2 separations of gases from non combustion gas streams, e.g. natural gas purification, re-breathers, contained environment CO2 concentration control and others.